Horizontal grave marker



FIG. 1 is a front perspective view of a horizontal grave marker showing my new design;

FIG. 2 is a right side elevation view (the left side elevational view is the same) of a horizontal grave marker showing my new design;

FIG. 3 is a plan view of a horizontal grave marker showing my new design; and,

FIG. 4 is a front elevation view (the rear elevational view is the same) of a horizontal grave marker showing my new design. 

The ornamental design for a horizontal grave marker, as shown and described. 